Integrated tunable filter architecture

ABSTRACT

An apparatus and method for a frequency based integrated circuit that selectively filters out unwanted bands or regions of interfering frequencies utilizing one or more tunable notch or bandpass filters or tunable low or high pass filters capable of operating across multiple frequencies and multiple bands in noisy RF environments. The tunable filters are fabricated within the same integrated circuit package as the associated frequency based circuitry, thus minimizing R, L, and C parasitic values, and also allowing residual and other parasitic impedance in the associated circuitry and IC package to be absorbed and compensated.

CROSS REFERENCE TO RELATED APPLICATIONS—CLAIM OF PRIORITY

The present application is a divisional of, and claims priority to, andcommonly assigned U.S. patent application Ser. No. 15/603,230, filed May23, 2017, entitled “Integrated Tunable Filter Architecture”, the entiredisclosure of which is incorporated herein by reference. applicationSer. No. 15/603,230 is a divisional of, and claims priority to, commonlyassigned U.S. patent application Ser. No. 14/181,332, entitled“Integrated Tunable Filter Architecture”, filed on Feb. 14, 2014, nowU.S. Pat. No. 9,673,155, issued Jun. 6, 2017, the entire disclosure ofwhich is incorporated herein by reference.

The present application may be related to the following patentapplications, assigned to the assignee of the present invention, theentire disclosures of which are incorporated herein by reference:

-   (1) U.S. patent application Ser. No. 12/735,954, Publication No.    2011000208A1, entitled “Method and Apparatus for Use in Digitally    Tuning a Capacitor in an Integrated Circuit Device”, filed on Mar.    2, 2009;-   (2) International Application No. PCT/US2009/001358, entitled    “Method and Apparatus for Use in Digitally Tuning a Capacitor in an    Integrated Circuit Device”, filed on Mar. 2, 2009;-   (3) U.S. patent application Ser. No. 13/595,893, entitled “Method    and Apparatus for Use in Tuning Reactance in a Circuit Device”,    filed on Aug. 27, 2012;-   (4) U.S. patent application Ser. No. 13/797,779 entitled “Scalable    Periphery Tunable Matching Power Amplifier”, filed on Mar. 3, 2013;-   (5) U.S. patent application Ser. No. 13/797,686 entitled “Variable    Impedance Match and Variable Harmonic Terminations for Different    Modes and Frequency Bands”, filed on Mar. 12, 2013;-   (6) U.S. patent application Ser. No. 13/828,121, entitled    “Autonomous Power Amplifier Optimization”, filed on Mar. 14, 2013;-   (7) U.S. patent application Ser. No. 13/829,946 entitled “Amplifier    Dynamic Bias Adjustment for Envelope Tracking, filed on Mar. 14,    2013;-   (8) U.S. patent application Ser. No. 13/830,555 entitled “Control    Systems and Methods for Power Amplifiers Operating in Envelope    Tracking Mode”, filed on Mar. 14, 2013;-   (9) U.S. patent application Ser. No. 13/967,866 entitled “Tunable    Impedance Matching Network”, filed on Aug. 15, 2013;-   (10) U.S. patent application Ser. No. 14/042,312, entitled “Methods    and Devices for Impedance Matching in Power Amplifier Circuits”,    filed on Sep. 30, 2013;-   (11) U.S. patent application Ser. No. 14/042,331 entitled “Methods    and Devices for Thermal Control in Power Amplifier Circuits”, filed    on Sep. 30, 2013;-   (12) U.S. patent application Ser. No. 11/520,912, entitled “Method    and Apparatus Improving Gate Oxide Reliability by Controlling    Accumulated Charge”, filed on Sep. 14, 2006;-   (13) U.S. patent application Ser. No. 11/881,816, entitled “Circuit    and Method for Controlling Charge Injection in Radio Frequency    Switches”, filed Jul. 26, 2007;-   (14) U.S. patent application Ser. No. 13/228,751, Publication No.    20130064064A1, entitled “Systems and Methods for Minimizing    Insertion Loss in a Multi-Mode Communications System”, filed on Sep.    9, 2011;-   (15) U.S. patent application Ser. No. 14/040,471 entitled “Antenna    Transmit Receive Switch”, filed on Sep. 27, 2013;-   (16) U.S. patent application Ser. No. 14/181,478 entitled “Methods    for Increasing RF Throughput Via Usage of Tunable Filters”, filed    Feb. 14, 2014;-   (17) U.S. patent application Ser. No. 14/181,489 entitled “Devices    and Methods for Duplexer Loss Reduction”, abandoned.-   (18) U.S. Pat. No. 6,667,506, entitled “Variable Capacitor with    Programmability”, issued on Dec. 23, 2003;-   (19) U.S. Pat. No. 6,804,502, entitled “Switch Circuit and Method of    Switching Radio Frequency Signals”, issued on Oct. 12, 2004;-   (20) U.S. Pat. No. 7,248,120, entitled “Stacked Transistor Method    and Apparatus”, issued on Jul. 24, 2007;-   (21) U.S. Pat. No. 7,910,993, entitled “Method and Apparatus for Use    in Improving Linearity of MOSFETS Using an Accumulated Charge Sink”,    issued on Mar. 22, 2011; and-   (22) U.S. Pat. No. 7,960,772, entitled “Tuning Capacitance to    Enhance FET Stack Voltage Withstand”, issued on Jun. 14, 2011.

BACKGROUND (1) Technical Field

This invention generally relates to electronic circuitry, and morespecifically to frequency based integrated circuits having at least oneintegrated tunable filter.

(2) Background

A large number of modern electronic systems, such as personal computers,tablet computers, wireless network components, televisions, cable system“set top” boxes, and cellular telephones, include frequency basedsubsystems such as radio frequency (RF) transceivers. Many of such RFtransceivers are actually quite complex two-way radios that transmit andreceive RF signals across multiple frequencies in multiple bands; forinstance, the 2.4 GHz band is divided into 14 channels spaced 5 MHzapart, beginning with channel 1 which is centered on 2.412 GHz. Asanother example, a modern “smart telephone” may include RF transceivercircuitry capable of concurrently operating on different cellularcommunications systems (e.g., GSM and CDMA), on different wirelessnetwork frequencies and protocols (e.g., IEEE 802.1bg at 2.4 GHz, andIEEE 802.1n at 2.4 GHz and 5 GHz), and on “personal” area networks(e.g., Bluetooth based systems).

In addition, such RF transceivers often operate in “noisy” RFenvironments, which includes other devices with RF transceivers (e.g.,wireless networks, cellular telephones and cell towers, and personalarea networks), as well as devices that emit electromagneticinterference on frequencies of interest (e.g., microwave ovens). Forexample, in the United States, devices that use the 2.4 GHz bandincludes wireless “WiFi” networks, microwave ovens, ISM band devices,security cameras, ZigBee devices, Bluetooth devices, video senders,cordless telephones, and baby monitors.

Designing frequency based electronic systems or subsystems capable ofoperating across multiple frequencies and multiple bands in noisyenvironments is a significant challenge, particularly in integratedcircuit solutions, which are desirable from a cost, reliability, size,and low power perspective. The apparatus and method described belowprovide a frequency based integrated circuit solution that selectivelyfilters out unwanted bands or regions of interfering frequenciesutilizing one or more integrated tunable notch or bandpass filters ortunable low or high pass filters. Various aspects of the apparatus andmethod described below will be seen to provide additional advantages.

SUMMARY OF THE INVENTION

The invention exemplified in the apparatus and method described belowprovides a frequency based integrated circuit solution that selectivelyfilters out unwanted bands or regions of interfering frequenciesutilizing one or more tunable notch or bandpass filters or tunable lowor high pass filters. The invention encompasses frequency basedelectronic systems or subsystems capable of operating across multiplefrequencies and multiple bands in noisy environments, particularly inintegrated circuit form, which is desirable from a cost, reliability,size, and low power perspective.

For RF applications in particular, it is important to carefully controlall aspects of the system R, L, and C components in order to minimizetheir impact on signal propagation. Such control is difficult orimpossible in many cases when a combination of integrated circuit (IC)circuits are combined with external R, L, and C components(collectively, “RLC” components). Accordingly, an important aspect ofthe invention is that the tunable filters (notch or low pass) arefabricated within the same IC package as the associated frequency basedcircuitry. In the examples shown in the accompanying figures, suchcircuitry comprises an RF switch, but other circuitry may be used. Suchintegration reduces or eliminates package and printed circuit board(PCB) RLC parasitic values, and also allows residual and other parasiticcapacitance, inductance, and resistance in the associated circuitry andpackage to be absorbed and compensated. For example, such integration,particularly if a digitally tunable capacitor is also integrated on thesame chip to provide the desired tunability, reduces parasiticcapacitances from such sources as IC pad shielding “cages”,electrostatic discharge (ESD) circuits, and the IC package itself.Accordingly, the invention encompasses co-designing a frequency basedcircuit and one or more tunable filters such that the overallperformance of the integrated combination is better than the simplecombination of separate components performing the same functions.

In one example embodiment, an RF switch is implemented on an integratedcircuit chip. The switch may be implemented with field effect transistor(FET) switch elements and is configured to be coupled to an externalantenna through a common port. At least two switch paths or “ports” ofthe switch are configured to be coupled to corresponding operational RFcircuits, such as RF transmitters and receivers, which may be fabricatedon the same IC chip as the switch or may be located on separate circuitstructures. At least one switch port is coupled to a bypassable tunablefilter (notch or low pass) integrated onto the same IC chip as theswitch. The tunability of the filter may be implemented using, forexample, a digitally tunable capacitor circuit and/or a tunableinductance circuit. The switch is designed to couple a selectedoperational RF circuit to a corresponding antenna while simultaneouslyselectively coupling an associated tunable filter to the same signalpath. Additional tunable filters (shunt or series type) may beaccommodated in similar fashion, so that multiple frequency bands may besimultaneously filtered out of an operational circuit signal path.

If sufficient circuit area is available, an embodiment may forego thetunability of individual notch filters and instead provide a number offixed frequency notch filters, utilizing the switching capability of theswitch to couple one or more of such notch filters to an operationalcircuit signal path, thus achieving “tunability” by notch filter circuitselection rather than by L or C component value changes.

For relatively high RF frequency ranges, such as the 2.4 GHz and 5 GHzRF bands commonly used in WiFi wireless local area networks, onlyrelatively small capacitance values are needed to achieve significantnotch frequency filtering. Such small values are well suited forimplementation on IC chips. For example, in one embodiment,approximately −17.2 dB of suppression centered at about 5.8 GHz can beattained with a value of 0.27 pF for the notch filter capacitance. Incontrast, attempting to achieve such a degree of notch frequencyfiltering with an external (off-chip) notch filter is extremelydifficult, if not impossible, since the necessary tuning capacitancevalue is small in comparison to all of the parasitic IC package andsurrounding circuit capacitances.

Notably, the insertion loss impact of an RF switch with integratedtunable notch or bandpass filters or tunable low or high pass filter canbe extremely small due to the ability to compensate for and absorbresidual and other parasitic capacitance, inductance, and resistance. Anoff-chip implementation of a filter would result in a significantlylarger insertion loss, typically in excess of 10 times larger than withembodiments of the present invention.

Variants of the illustrated embodiments include use of one or more shuntand/or series tunable filters, use of differential filters, placement oftunable filters closer or farther from an associated antenna, use ofcompensation capacitors when a tunable filter is bypassed, addition ofinductors for overall circuit tuning purposes, and other aspectsdescribed below.

Other variants of the inventive concept that provide additionalfunctionality and flexibility include the use of tunable low passfilters on the port side of an RF switch, and a tunable RF duplexerincorporating both a tunable low pass filter and a tunable high passfilter.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a typical notch filter as embodied inpractice.

FIG. 2A is a schematic diagram of a specific tunable notch filtercircuit.

FIG. 2B is a schematic diagram of a differential configuration of atunable notch filter.

FIG. 3 is a diagram of a spiral inductor structure.

FIG. 4 is a graph of attenuation versus frequency for the notch filterdepicted in FIG. 2A.

FIG. 5 is a block diagram of an RF switch implemented on an integratedcircuit chip in accordance with one aspect of the present invention.

FIG. 6 is a graph of attenuation versus frequency for variouscapacitance values C of a tunable notch filter in accordance with oneaspect of the present invention.

FIG. 7 is a graph of attenuation versus frequency for a pair of notchfilters of the type depicted in FIG. 2A.

FIG. 8 is a schematic RLC-model diagram of tunable notch filter in an“OFF” state, integrated with an RF switch circuit in an IC chip inaccordance with one aspect of the present invention.

FIG. 9 is a graph of attenuation versus frequency for an embodiment of atunable notch filter integrated with an RF switch of the type and statedepicted in FIG. 8.

FIG. 10 is a schematic RLC-model diagram of the circuit of FIG. 8 withthe tunable notch filter in an “ON” state.

FIG. 11 is a graph of attenuation versus frequency for an embodiment ofa tunable notch filter integrated with an RF switch of the type andstate depicted in FIG. 10.

FIG. 12 is a schematic diagram of a low pass filter structure suitablefor use with the present invention in place of the tunable notch filtershown in FIG. 5.

FIG. 13 is a graph of attenuation versus frequency for the low passfilter structure depicted in FIG. 12.

FIG. 14 is a block diagram of another embodiment of a pair of RFswitches implemented on an integrated circuit chip in accordance withaspects of the present invention.

FIG. 15 is a schematic circuit diagram of one embodiment of a bypassableseries tunable notch filter.

FIG. 16 is a schematic RLC-model diagram of one embodiment of aswitching circuit that includes a tunable filter and bypass switch, anadded bypassable compensation capacitor, and an added tuning inductor.

FIG. 17 is a block diagram of an RF switch configuration implemented onan integrated circuit chip in accordance with one aspect of the presentinvention.

FIG. 18 is a schematic diagram of one embodiment of a tunable low passfilter circuit that may be used for the tunable low pass filter elementsshown in FIG. 17.

FIG. 19 is a block diagram of a shared tunable low pass filter for an RFcircuit configuration in accordance with one aspect of the presentinvention.

FIG. 20 is a block diagram of one embodiment of a tunable RF duplexerincorporating both a tunable low pass filter and a tunable high passfilter.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION OF THE INVENTION

The invention exemplified in the apparatus and method described belowprovides a frequency based integrated circuit solution that selectivelyfilters out unwanted bands or regions of interfering frequenciesutilizing one or more tunable notch or bandpass filters or tunable lowor high pass filters. The invention encompasses frequency basedelectronic systems or subsystems capable of operating across multiplefrequencies and multiple bands in noisy environments, particularly inintegrated circuit form, which is desirable from a cost, reliability,size, and low power perspective.

For RF applications in particular, it is important to carefully controlall aspects of the system R, L, and C components in order to minimizetheir impact on signal propagation. Such control is difficult orimpossible in many cases when a combination of integrated circuit (IC)circuits are combined with external R, L, and C components. Accordingly,an important aspect of the invention is that the tunable notch orbandpass filters or tunable low or high pass filters are fabricatedwithin the same integrated circuit (IC) package (comprising one or moreIC “chips”, coupled by IC package circuit traces, bond wires, wirelesscommunication circuitry, etc.) as the associated frequency basedcircuitry. In particular, monolithic integration reduces package andprinted circuit board (PCB) R, L, and C parasitic values, and alsoallows residual capacitance and inductance (i.e., capacitance andinductance from passive elements such as resistors, capacitors, andinductors) and other parasitic capacitance, inductance, and resistance(“parasitic impedance”) in the associated circuitry and package to beabsorbed and compensated, thus eliminating or reducing the impact onsignal propagation of such capacitances.

More generally, whenever there exists “parasitic” (not intentionallydesigned) capacitance or inductance (such as the inherent capacitance ininductor structures and inductance in capacitor structures, or packagecapacitance and inductance approaching the size of desired circuitvalues), the parasitic capacitance or inductance can usually bedescribed as an additional capacitor or inductor in the equivalentcircuit model. If it appears in series or parallel with anintentionally-designed element of the same type (capacitor or inductor),the value of the intentionally designed component can be modified(usually reduced) such that the effective value (intentional andparasitic portions combined) is restored to the originally intendedvalue. This can be done only if the parasitic contributions are smallenough so as not to exceed the desired value, which will typically bethe case only if the circuit and packaging are integrated and/orco-designed.

FIG. 1 is a schematic diagram of a typical fixed-value notch filter 100as embodied in practice. As is known in the art, a notch filter is atype of band-stop or band-rejection filter that suppresses a relativelynarrow band of frequencies. One type of notch filter simply comprises aninductor L and a capacitor C coupled in series between a signal path andcircuit ground, as shown in FIG. 1. In practical implementations, LCnotch filters also inherently include at least a “parasitic” seriesresistance R, as shown, thus resulting in a series “RLC” structure.Importantly, the parasitic resistance R, the total capacitance C, andthe total inductance L are determined by other coupled or nearbycomponents, and not just by the purposely fabricated L and C elements ofthe notch filter 100. For example, the capacitance C of the circuitshown in FIG. 1 comprises not only the value of a specific capacitordesigned into the circuit, but also parasitic capacitances from coupledor surrounding components, such as the inductor L. Similarly, theinductance L comprises not only the value of a specific inductordesigned into the circuit, but also parasitic inductances from coupledor surrounding components. In the same manner, the parasitic resistanceR of the circuit shown in FIG. 1 includes the resistance of the inductorL, interconnecting wire or circuit traces, and any switch (e.g., FETtransistor switches) element in the circuit. Accordingly, it isimportant in designing frequency based circuitry to account for andaccommodate or compensate for such parasitic circuit elements in orderto achieve desired specifications.

It should be noted that while the R and C elements shown in FIG. 1 aretypically considered to be passive elements, each may be implemented byusing FETs configured to behave as resistors and/or capacitors, in knownfashion.

FIG. 2A is a schematic diagram of a specific tunable notch filtercircuit 200. In this particular example, R1 and R2 represent smallparasitic resistances (e.g., about 0.01 Ohms each) in series betweenports P1 and P2. The rest of the tunable notch filter circuit comprisesa main inductor L1 (shown in this example as a spiral structure,discussed below, having a value of about 2.4 nH), a resistor R3 (about 3Ohms in this example, comprising the resistance of the inductors and acoupled switch element), and a capacitor C (about 0.27 pF in thisexample, a notably small value). Also shown is a parasitic inductor L2(having a value of about 0.4 nH in this example). To make the notchfilter 200 tunable, either or both of the inductor L1 or the capacitor Cmay be adjustable over a range of values, as indicted by the dottedarrows in FIG. 2A.

An example of a suitable adjustable capacitor for C in FIG. 2A is thedigitally tunable capacitor (DTC) shown in U.S. patent application Ser.No. 12/735,954, Publication No. 2011000208A1, entitled “Method andApparatus for Use in Digitally Tuning a Capacitor in an IntegratedCircuit Device”, filed on Mar. 2, 2009 and assigned to the assignee ofthe present invention. Utilizing a variable capacitor, the notch filtershown in FIG. 2A can have its center suppression frequency varied over aconsiderable range of values by tuning the capacitance C included aspart of the RLC notch filter circuit 200 to a desired value.

Similarly, the center suppression frequency of the RLC notch filtercircuit 200 can be varied by using a tunable inductor circuit (e.g., aspiral inductor L having multiple sections which can be switched in orout of circuit to alter the circuit inductance, or multiple inductorspirals that may be selectively switched into or out of the RLC notchfilter circuit to change the total inductance). In addition, a tunablenotch filter may be implemented having both a variable capacitor and avariable inductor, which may provide a wider range of tuning.

FIG. 2B is a schematic diagram of a differential configuration of atunable notch filter 202. The tunable notch filter 202 includes anL-C-C-L series circuit between differential signal paths 204 and 206 (intheory, the two capacitors C could be combined into one, but practicalconsiderations for IC implementations often require a pair of capacitorstructures; note also that parasitic resistances have been omitted forclarity). While all four notch filter elements are shown as tunable,only a subset (typically both capacitors C) need be tunable for mostapplications. A differential implementation is beneficial in mitigatingparasitics such as ground inductance.

FIG. 3 is a diagram of a planar spiral inductor structure 300 that maybe useful in implementing the notch filter depicted in FIG. 2A. Such astructure is particularly adapted to being fabricated on an integratedcircuit. Typically, port 1 in the center of the inductor structure 300is coupled to ground through a capacitor (not shown), and port 2 isconnected to a signal path; the structure thus behaves as a shuntelement to ground. However, the inductor structure 300 may also be usedas a series connected element (see, for example, the discussion belowregarding FIG. 15).

Structure parameters for a spiral inductor structure 300 that affect theinductance value include line width, line pitch, and number of turns, asis known in the art. The general shape of a spiral inductor may be, forexample, circular, octagonal, hexagonal, square, or other suitablegeometric form. The spiral inductor structure 300 may be made tunable byincluding multiple “taps” to portions of the spiral between illustratedport 1 and port 2, accessed by a switching structure (not shown) thatenables connectivity of a subset of the spiral coils to a signal path.Alternatively, and sometimes preferably, other parts of a circuit may bestatically coupled to port 1 and port 2 of the spiral structure, but oneor more switches may be configured to short out part of the spiralinductor structure by connecting adjacent turns of spiral using alow-impedance “ON” state, thus providing tunability.

FIG. 4 is a graph of attenuation versus frequency for the specific notchfilter circuit 200 depicted in FIG. 2A (i.e., with the specificcomponent values set forth above). At point m1, the signal power loss (Sparameter) from port P1 to port P2 is only about −0.06 dB at about 1.973GHz, a quite small amount. In contrast, at the center frequency m2 ofthe notch filter circuit 200, the signal power loss from port P1 to portP2 is about −17.2 dB at about 5.8 GHz. Accordingly, a range offrequencies centered around about 5.8 GHz are effectively suppressed orfiltered out of the signal path. As is known in the art, the shape ofthe notch may be altered by adding additional series inductance to thenotch filter circuit, which produces a “sharper” or more abrupt“shoulder” 400 as more inductance is added. As another example, for thetunable notch filter 200 of FIG. 2A, using a larger L1 and smaller C(keeping the L1*C product—and therefore the notch frequency—constant)results in a shallower notch and lower insertion loss at the “shoulder”400. Larger C and smaller L1 values deepen the notch and increases theinsertion loss at the “shoulder” 400. (Note: for the series tunablenotch filter 1500 configuration shown in FIG. 15, the opposite resultoccurs when varying the L and C components).

Frequency-based integrated circuitry is found in a variety ofapplications, such as modern cellular telephones. In recent years, thecomplexity of cellular telephones has increased rapidly, moving fromdual-band to tri-band, and more recently, to quad-band. In addition,cellular telephones need to be able to accommodate a variety of RFsignals for peripheral radios, such as Bluetooth, Wi-Fi, and GPS. Thistrend is expected to continue as other RF capabilities are added;current “smart phone” cellular telephone architectures have at leastseven radios in a single handset. Complexity will continue to rise dueto the increased popularity of peripheral radios and functions that alsoneed access to the antenna of a handset. The increased complexity inmobile telephone handset design has greatly complicated the RF front-endby more than tripling the number of high-power signal paths. By itsnature, a multiband handset must accommodate a plurality of RF signalpaths that all operate on different center frequencies and bandwidths.Yet in many designs, all of the RF signal paths must share access to asmall number of antennas (often only one or two). Accordingly, a veryefficient solution is to route all of the competing RF signal paths toone or two antennas using an RF switch implemented on an integratedcircuit chip.

FIG. 5 is a block diagram of an RF switch 502 implemented on anintegrated circuit chip 500 in accordance with one aspect of the presentinvention. The switch 502 may be implemented with FET switch elements asshown in U.S. Pat. No. 6,804,502, issued Oct. 12, 2004 entitled “SwitchCircuit and Method of Switching Radio Frequency Signals” and assigned tothe assignee of the present invention. In the illustrated embodiment, anantenna 504 is electrically coupled to a common port of switch 502,optionally through a bypassable low pass filter 506 (discussed below).With one antenna 504, the illustrated embodiment is configured foroperation across one radio frequency range or “band” typicallycomprising N sub-bands (which may or may not be adjacent in the RFspectrum encompassed by the band). However, the invention encompassesembodiments that include multiple antennas coupled to corresponding RFswitches 502 monolithically integrated onto a single IC chip 500 so asto be operable across multiple radio frequency bands.

In the illustrated embodiment, N signal switch paths or “ports” of theswitch 502 are shown coupled to corresponding operational RF circuits512. The operational RF circuits 512 represent other circuitry, such asRF transmitters and receivers, that may be fabricated on the same ICchip as the switch 502, or that may be located on separate circuitstructures (shown as off-chip in FIG. 5). Any of the RF circuits 512coupled to the ports of the RF switch 502 may be selectively coupled tothe antenna 504 in order to transmit and/or receive signals on acorresponding RF sub-band.

At least one switch port of the switch 502 is coupled to a tunable(e.g., via a DTC circuit and/or a tunable inductance circuit) notchfilter 510. The switch 502 is designed to couple a selected operationalRF circuit 512 to the antenna 504 by activating corresponding switchelements SW1-SWN while simultaneously selectively coupling theassociated tunable notch filter 510 to the same signal path through acorresponding switch element SW0. For example, when RF Circuit 2 iscoupled by the switch 502 to the antenna 504, switch element SW2 wouldbe closed. If it is desirable to also couple the tunable notch filter510 to the same signal path in order to provide notch filteringfunctionality, then switch element SW0 also would be closed. Since theswitch 502 needs to be configured to couple at least two ports(including the port coupled to the tunable notch filter 510) to thecommon port, that functionality may be generalized so that the switch502 may concurrently couple any combination of the signal ports to thecommon port.

A number of variants of the integrated circuit chip 500 may beimplemented. For example, while only one tunable notch filter 510 isshown coupled to the switch 502 in FIG. 5, additional tunable notchfilters 510 may be accommodated in similar fashion, so that multiplefrequency bands may be simultaneously filtered out of an operationalcircuit signal path. In addition, one or more series tunable notchfilters may be used in lieu of one or more shunt tunable notch filters,as described in further detail below with respect to FIG. 15. Further,if sufficient circuit area is available, an embodiment may forego thetunability of individual notch filters and instead provide a number offixed frequency notch filters coupled to multiple ports of a switch 502.Utilizing the switching capability of the switch 502, one or more ofsuch notch filters may be coupled to an operational circuit signal path,thus achieving “tunability” by notch filter circuit selection ratherthan by tunable component value changes. Accordingly, as defined withrespect to the present invention, a “tunable notch filter” includes astructure of multiple selectable integrated fixed frequency notchfilters.

For relatively high RF frequency ranges or bands, such as the 2.4 GHzand 5 GHz RF bands commonly used in WiFi wireless local area networks,only relatively small capacitance values are needed to achievesignificant notch frequency suppression. Such small values are wellsuited for implementation on IC chips. For example, utilizing a circuitsimilar to that shown in FIG. 2A, but with an integrated DTCimplementation of the capacitor C, approximately −17.2 dB of frequencysuppression centered at about 5.8 GHz can be attained with a (total)value of 0.27 pF for C. In contrast, attempting to achieve such a degreeof notch frequency suppression with an external (off-chip) notch filteris extremely difficult, if not impossible, since the necessary tuningcapacitance value is small in comparison to all of the parasitic packageand circuit capacitances.

While FIG. 5 shows the tunable notch filter 510 as being integratedwithin the structure of the integrated circuit chip 500 on the signalpath side of the switch 502 (i.e., ports 1-N), it may also beadvantageous to use one or more integrated tunable notch filters (orbypassable low pass filters, as discussed below) on the common port(antenna) side of the switch. Placing integrated tunable notch filterson the antenna side would allow absorption of residual notch filtercapacitance in the switch 502, thus reducing overall signal power loss.Additional inductance may be coupled to the tunable notch filter tobetter match the impedance of the filter to the circuit as a whole.

FIG. 6 is a graph of attenuation versus frequency for variouscapacitance values C of a tunable notch filter in accordance with oneaspect of the present invention. Each depicted graph curve is similar tothe single graph curve shown in FIG. 4, but all of the depicted curvesare achievable using a single tunable notch filter with a variablecapacitor, such as a DTC, to vary the values of C in an RLC circuit ofthe type shown in FIG. 2A. As should be apparent from FIG. 6, by havingmultiple tunable notch filters incorporated into an integrated circuitchip 500 as shown in FIG. 5, multiple frequency notches can be achievedconcurrently. This may be important in many applications where asystem's own transmit signal on one band must be suppressed for thereceive side of the system concurrently with filtering out environmental“noise” on other received bands.

As noted above, multiple notch filters may be configured as part of oneor more RF switches 502 of the type shown in FIG. 5. If two or more suchnotch filters are configured in parallel with close center suppressionfrequencies (e.g., by using close values of C), the effect is to widenthe notch of filtered frequencies. For example, FIG. 7 is a graph ofattenuation versus frequency for a pair of notch filters of the typedepicted in FIG. 2A, with C=0.215 pF selected for the first notch filterand C=0.195 pF selected for the second notch filter. Comparing thisgraph to the graph shown in FIG. 4 (where C in the example notch filterhad a value of 0.27 pF) shows that the width of the notch of suppressedfrequencies has been widened due to the existence of two minima, m2 andm3. As the difference in C values grows, the paired notch filters willsuppress different (and eventually non-overlapping) ranges offrequencies. A similar effect can be achieved by varying the inductancevalue of the pair of RLC notch filters. Of course, more than two notchfilters may be implemented to provide an even wider frequencysuppression notch and/or to provide multiple separate and/or overlappingfrequency suppression notches.

FIG. 8 is a schematic RLC-model diagram of tunable notch filter in an“OFF” state, integrated with an RF switch circuit 800 in an IC chip inaccordance with one aspect of the present invention. An active (“ON”)port P1 for the switch 800 comprises a switch (e.g., a FET) that ismodeled as an RLC network in block 802. The values of the modeledcomponents will of course vary with the application and circuitspecifications. A common port P2 for the switch 800, used for examplefor connections to an antenna, is shown with LC components in block 804for tuning out the P1 to P2 circuit capacitance for matching purposes;the inductor L may be off-chip if need be.

In the illustrated embodiment, a tunable notch filter 806 that includesan inductor 807 (shown as a spiral inductor in this example) and atunable capacitor 808 may be coupled to the P1-P2 signal path by meansof a selective switching structure 809, shown here modeled asalternative RC pathways which may be implemented, for example, as FETs.In the configuration shown in FIG. 8, the tunable notch filter 806 is“OFF”: circuit path 809 a is switched into connection with the P1-P2signal path, and circuit path 809 b (shown in dotted lines) is switchedout of connection with the P1-P2 signal path. The result is that thetunable notch filter 806 is shunted to ground and the switchingstructure 809 appears as an open circuit to the P1-P2 signal path (i.e.,at RF frequencies, the active capacitor in block 809 behaves as an opencircuit). (For clarity, note that the selective switching structure 809comprises only two components, each of which behaves as either aresistance or a capacitance based on the state of a control signal, notshown. Circuit path 809 a in FIG. 8 shows one state of the twocomponents; circuit path 809 b in FIG. 10 shows the other state of thetwo components).

Also attached to the P1-P2 signal path are the “OFF” (non-active) ports810 of the RF switch circuit 800. Importantly for practical concerns,all of the “OFF” ports of the switch 800, while theoreticallydisconnected from the P1-P2 signal path, in fact still present aparasitic RLC network load due to the nature of semiconductor ICfabrication. For example, as noted above, the switch 800 elements may beimplemented with FETs. However, “open”, non-conductive FETs stillpresent a measurable capacitance to coupled circuits. In the embodimentillustrated in FIG. 8, for an N-port switch 800 with one active port andonly one tunable notch filter 809, there are N−2 “OFF” ports. Animportant aspect of the present invention is that integrating one ormore tunable notch filters within an RF switch allows optimization ofboth the RF switch and the tunable notch filters so that the residualand other parasitic impedance in the circuitry and package associatedwith both parts can be synergistically absorbed and compensated. Forexample, the actual capacitor structures needed for the RLC notch filtercan be made smaller than the desired effective capacitance values,thereby “absorbing” the parasitic capacitance of the associated switchcircuitry and IC package, which add to the total value of C for thenotch filter structure. As another example, compensation for the RFswitch 800 shown in FIG. 8 is achieved by properly sizing the inductorsin blocks 802 and 804 for a particular application to offset or “match”the capacitance of the “OFF” ports in block 810 and of the tunable notchfilter and bypass switch components in blocks 806 and 809, so that theP1 to P2 signal path looks like a 50-ohm transmission line. Suchcompensation reduces the insertion loss impact of the RF switch 800.

FIG. 9 is a graph of attenuation versus frequency for an embodiment of atunable notch filter integrated with an RF switch of the type and state(i.e., notch filter “OFF) depicted in FIG. 8. The signal power loss (Sparameter) from port P1 to port P2 increases gradually with frequency,but no notch filter function is apparent.

FIG. 10 is a schematic RLC-model diagram of the circuit of FIG. 8 withthe tunable notch filter in an “ON” state. In this example, the tunablenotch filter 806 is “ON”: circuit path 809 a (shown in dotted lines) isswitched out of connection with the P1-P2 signal path, and circuit path809 b is switched into connection with the P1-P2 signal path. The resultis that the tunable notch filter 806 is resistively coupled to the P1-P2signal path, thus permitting notch frequency suppression.

FIG. 11 is a graph of attenuation versus frequency for an embodiment ofa tunable notch filter integrated with an RF switch of the type andstate depicted in FIG. 10. The signal power loss (S parameter) from portP1 to port P2 increases gradually with frequency until about 5 GHz,where the notch filter function becomes readily apparent. Notably, inthe embodiment represented by FIG. 11 and with the values used toachieve the illustrated curve characteristics, the insertion loss impactof the RF switch with integrated tunable notch filter is only about 0.03dB (i.e., the difference between the loss at m1 in FIG. 9 and the lossat m1 in FIG. 11), due to the ability to compensate for and absorbresidual and other parasitic inductance. An off-chip implementation of anotch filter would result in a significantly larger insertion loss,typically in excess of 0.5 dB, more than a factor of 10 larger than withembodiments of the present invention.

The embodiments shown in FIGS. 8 and 10 allow the tunable notch filter806 to be actively switched onto or off of the P1-P2 signal path asneeded by suitable control circuitry (not shown). In an alternativeembodiment, the tunable notch filter 806 may remain electricallyconnected to the P1-P2 signal path, but have its filteringcharacteristics tuned (e.g., by using a DTC and minimizing itscapacitance) as needed by suitable control circuitry to move its centersuppression frequency to a band that is outside the range of interestfor a particular RF system (in effect, “detuning” the filter). A furtherembodiment allows for tuning the notch filter to a minimum capacitancestate when it is in its OFF state (i.e., when the switch port to thenotch filter is not engaged), in order to reduce losses through thefilter RLC structure. Accordingly, a tunable notch filter may be eitherswitched ON (electrically connected) while physically connected to asignal path in order to perform filtering, or functionally activatedwhile constantly electrically and physically connected to a signal pathin order to perform filtering.

As should be apparent from FIG. 11, if the only frequencies beingtransmitted or received (e.g., around point m1) are solely in a lowerrange than interfering frequencies, an integrated low pass filter can beused in place of the notch filter shown in FIG. 5. FIG. 12 is aschematic diagram of a low pass filter structure 1200 suitable for usewith the present invention in place of the tunable notch filter 510shown in FIG. 5. Shown is a conventional 3-element CLC low pass filter1202. A 5-element LC low pass filter can be implemented by addinganother LC stage 1204, and a 7-element LC low pass filter can beimplemented by adding another LC stage 1206. The low pass filterstructure 1200 is optionally tunable. For example, one or more of thecapacitors in each stage may be implemented with a DTC to allow tuningof the cut-off frequency of the low pass filter. In an alternativeembodiment, one or more of the inductors in the LC stages may be tunableto allow tuning of the cut-off frequency of the low pass filter. Thebypass switch 1208 and the shunt switches 1210 allow the low pass filterstages to be enabled (by opening bypass switch 1208 and closing theshunt switches 1210) or bypassed (by closing bypass switch 1208 andopening the shunt switches 1210).

FIG. 13 is a graph of attenuation versus frequency for the low passfilter structure 1200 depicted in FIG. 12. As can be seen, low passfilters with more elements provide a more abrupt cut-off of highfrequencies, at the cost of additional components. As with the tunablenotch filter, a low pass filter would normally be switched off of theP1-P2 signal path when not in use. Similarly, a low pass filter may betuned to reduce loss when not in use by moving the cut-off point to afrequency band outside the range of interest. The resulting integratedswitch and filter structure also exhibits low insertion losses.

FIG. 14 is a block diagram of another embodiment of a pair of RFswitches implemented on an integrated circuit chip 1400 in accordancewith aspects of the present invention. The illustrated circuit issimilar to the circuit shown in FIG. 5, but includes two RF switches1402 a, 1402 b coupled by means of common ports to correspondingantennas 1404 a, 1404 b through associated bypassable low pass filters1406 a, 1406 b. This is a similar configuration to one embodimentdescribed above in which tunable notch filters are situated on thecommon (antenna) side of their corresponding RF switch. The RF switches1402 a, 1402 b can selectively couple one or more associated RF circuits1412 a, 1412 b to an associated antenna 1404 a, 1404 b. The bypassablelow pass filters 1406 a, 1406 b are selectively enabled or bypassed inthe manner described with respect to FIG. 12. If needed for impedancematching, one or more inductors (not shown) can be switched into theoperational signal path when a low pass filter 1406 a, 1406 b isbypassed.

Variants of the invention include combining multiple aspects of thedescribed embodiments. For example, referring to FIG. 5, a bypassablelow pass filter of the type shown in FIG. 12 can be included in a switchcircuit of the type shown in FIG. 5. In particular, an optionalbypassable low pass filter 506 is shown coupled to the common port ofthe RF switch 502. When engaged, the bypassable low pass filter 506 canfilter out a broad range of interfering high frequencies that mayadversely affect all of the RF circuits 512, while the tunable notchfilter 510 can filter out selected frequency bands within the lowerrange of frequencies admitted by the low pass filter 506. If desired,the bypassable low pass filter 506 may also be made tunable.

As another variant of the circuit shown in FIG. 5, the optionalbypassable low pass filter 506 may be replaced by a bypassable seriestunable notch filter, and the tunable notch filter 510 may be omitted oralternatively used in combination with the bypassable series tunablenotch filter. In an alternative configuration, one or more signal portsmay include a dedicated bypassable series tunable notch filter.

FIG. 15 is a schematic circuit diagram of one embodiment of a bypassableseries tunable notch filter 1500 which is preferable to the shunt notchfilter circuit 200 shown in FIG. 2A in some applications. A tunablenotch filter element 1501 comprising a primary inductor 1502, acapacitor 1503, and a residual matching inductor 1506 coupled as shownoperates in a manner similar to the notch filter of FIG. 2A to suppressa narrow range of frequencies. Either or both of the primary inductor1502 or the capacitor 1503 are tunable in order to select a centersuppression frequency. In the illustrated embodiment, the capacitor 1503is shown as tunable, and may be, for example, a DTC of the typedescribed in U.S. patent application Ser. No. 12/735,954 cited above. Asanother example, a multi-tap tunable inductor as described above withrespect to FIG. 3 may be used for the primary inductor 1502 of thetunable notch filter element 1501, series connected as shown.

In FIG. 15, an inductor bypass switch 1504 and a notch enable switch1508 allow the notch filter element 1501 to be effectively removed fromthe circuit if no filtering function is needed. The inductor bypassswitch 1504 is OPENED and the notch enable switch 1508 is CLOSED whenthe notch filter element 1501 is to be operative (enabled). When thenotch filter element 1501 is to be inoperative (disabled, i.e., nofiltering), the inductor bypass switch 1504 is CLOSED and the notchenable switch 1508 is OPENED. As a practical matter, the bypass switch1504 and the notch enable switch 1508 should have a low resistance when“ON” so as to maintain low insertion loss. When the notch filter element1501 is disabled, the primary inductor 1502 is shorted and the capacitor1503 is disconnected. In that condition, the residual matching inductor1506 remains in the circuit path between the switch side and the antennaside. The residual matching inductor 1506 should be sized appropriatelyto emulate a 50-ohm transmission line and minimize insertion loss whenthe notch filter element 1501 is disabled. When the notch filter element1501 is enabled, both the primary inductor 1502 and the residualmatching inductor 1506 form part of the tunable notch filter element1501, along with the capacitor 1503. With the capacitor 1503 thusconnected, the primary inductor 1502 and the residual matching inductor1506 are sized appropriately to emulate a 50-ohm line and minimizeinsertion loss at the signal transmission frequency.

For particular implementations of the embodiments of the invention,special care may be taken to mitigate the influence of added tunablefilter components. For example, FIG. 16 is a schematic RLC-model diagramof one embodiment of a switching circuit 1600 that includes a tunablefilter 1602 and bypass switch 1604, an added bypassable compensationcapacitor 1606, and an added tuning inductor 1608. The switching circuit1600 is similar to the circuits shown in FIG. 8 and FIG. 10, butsimplified to show a tunable filter 1602 (e.g., a tunable notch filteror a tunable low pass filter) as a single element. For applicationsparticularly sensitive to impedance matching, when the tunable filter1602 is bypassed by opening switch 1604, an optional second switch 1610concurrently closes to couple the optional compensation capacitor 1606onto the same signal path. Doing so ensures that the capacitance toground, when the tunable filter 1602 is disconnected, is the same as theeffective capacitance to ground at the transmitted frequency, when thetunable filter 1602 is connected. This enables minimum insertion loss(in a 50-ohm system) by emulating a 50-ohm transmission line, regardlessof the connected or disconnected state of the tunable filter 1602.

More particularly, a perfect (distributed) 50-ohm line is characterizedby sqrt(L/C)=50 ohms, where L and C are the inductance and capacitanceper unit length, respectively. In a circuit having lumped inductance Land capacitance C, minimum-loss transmission in a 50-ohm system is stillachieved when sqrt(L/C) is on the order of 50 ohms. In FIG. 16, theinductance of the signal path resides primarily in the combination oftuning inductor 1608, the inductor L within block 804, and theinductance of active port 802. When activated by bypass switch 1604,tunable filter 1602 is almost a zero impedance at the filter frequency,but is capacitive at the frequency being transmitted. The inductancevalues of tuning inductor 1608 and the inductor L within block 804 arechosen to give a 50-ohm line at the frequency being transmitted, withthe tunable filter 1602 activated. To preserve a 50-ohm line when thetunable filter 1602 is not activated, closing switch 1610 insertscompensation capacitor 1606 onto the signal path. The value ofcompensation capacitor 1606 is selected to match—and thus substitutefor—the effective capacitance of the tunable filter 1602 at thetransmitted signal frequency when the tunable filter 1602 is activated.

FIG. 16 also shows an optional tuning inductor 1608 that may be added tohelp tune out excess capacitance in the switching circuit 1600. Doing somakes it easier to tune the P1-P2 signal path to be a near-ideal 50-ohmtransmission line.

It may be beneficial in some applications to physically locate a tunablefilter (notch or low pass) closer to an antenna port. It may also beadvantageous in some applications to provide a bypass switch for such atunable filter that is separate from the RF switch (element 502 in FIG.5) but still integrated on the same IC die or in the same IC package.Among other advantages, such a configuration enables easy adaptation ofthe inventive concepts to existing circuit designs for such RF switches.

Another variant of the inventive concept that provides additionalfunctionality and flexibility includes the use of tunable low passfilters on the port side of an RF switch. For example, FIG. 17 is ablock diagram of an RF switch configuration implemented on an integratedcircuit chip 500 in accordance with one aspect of the present invention.The integrated circuit chip 500 is essentially the same as the circuitof FIG. 5, with the addition of one or more tunable low pass filters(TLPF) 1702 ₁-1702 _(N) formed on the IC chip 500 before one or morecorresponding switch elements SW1-SWN. The TLPF's help filter outharmonic frequencies from an associated RF signal path. In someapplications, only two of the ports may need TLPFs; for example, in GSMradio systems, TLPF's may only be desirable on a low band RF path and ahigh band RF path. Accordingly, the remaining RF circuit paths need nothave TLPFs added on the integrated circuit chip 500.

FIG. 18 is a schematic diagram of one embodiment of a tunable low passfilter circuit 1800 that may be used for the tunable low pass filterelements shown in FIG. 17. In the illustrated embodiment, inductors 1802a-1802 c and capacitors 1804 a-1804 c form a variant of the well-knownshunt-serial-shunt “pi” type low pass filter, and tunability of theillustrated filter circuit is achieved by using variable capacitors. Forexample, one or more of the capacitors 1804 a-1804 c may be implementedwith a DTC to allow tuning of the cut-off frequency of the tunable lowpass filter 1800. In addition, in the illustrated circuit, adjusting thecapacitors 1804 a-1804 c (not necessarily by the same amount) serves notonly to tune the cut-off frequency, but also the “notches” at harmonicfrequencies (for example, the 2nd and 3rd harmonics in this example)beyond the cut-off frequency to provide harmonic suppression. In analternative embodiment, one or more of the inductors 1802 a-1802 c maybe tunable to allow tuning of the cut-off frequency of the low passfilter. As an option that may be useful for some applications, additionof a bypass switch 1806 and a pair of shunt switches 1808 allows thetunable low pass filter to be enabled (by opening bypass switch 1806 andclosing the shunt switches 1808) or bypassed (by closing bypass switch1806 and opening the shunt switches 1808). If the shunt switches 1808are not included, the corresponding inductors 1802 a, 1802 c would beconnected directly to circuit ground in the illustrated circuit. Asshould be apparent to those skilled in the art, other low pass filterdesigns may be useful for various applications, so long as they aretunable. While FIG. 18 shows a typical shunt-serial-shunt “pi” type LPF,low pass filter designs are not limited to shunt-serial-shuntarchitectures. For example, a simple serial-shunt L-type LPF may beused, like stages 1204 and 1206 shown in FIG. 12.

Existing circuits known as “antenna switch modules” (ASMs) include an RFswitch with harmonic filters. However, the filters are each designed fora specific frequency band, and therefore multiples of such filters arerequired in a system that transmits or receives signals in multiplefrequency bands. Further, process variations (MIM capacitance value,bond-wire length, etc.) during the manufacture of ASMs cause shifts infiltering parameters, degrade product quality, and increase designchallenges. In particular, having to design around such processvariations leads to long product-to-market time and higher engineeringcosts due to multiple design iterations to optimize filter performance.

In contrast, integrating one or more TLPF's and an RF switch into anintegrated circuit chip 500, as shown in FIG. 17, results in an ASM thatallows post-manufacture tuning of each port equipped with a TLPF and thebroader ability to handle multiple frequency bands. For processvariation tuning, a relatively small range of tunability may besuitable, and may be accomplished, for example, by varying bias inputsof the tunable capacitors of the TLPFs (see FIG. 18) via a digitalinterface to the integrated circuit chip 500 so that each TLPF can becontrolled individually. In some cases, it may be desirable to allow thecustomer to do all calibration and tuning when the integrated circuitchip 500 is embedded in a final circuit, in order to optimize the tuningof the integrated circuit chip 500 to take into account the variousexternal parasitic RLC elements of the complete environment in which thechip is embedded. A customer could also customize or tune the amount ofattenuation needed for a particular radio (with the radios different bydesign or different due to process or part variations).

If tunable capacitors or tunable inductors with a sufficiently widetuning range are incorporated into a TLPF, so that the TLPF has a widerange of variability in its cut-off frequency, then a single TLPF may beused in conjunction with a simple switch to provide harmonic filteringof multiple RF bands. For example, FIG. 19 is a block diagram of ashared tunable low pass filter for an RF circuit configuration 1900 inaccordance with one aspect of the present invention. In the illustratedembodiment, a low band RF circuit 1902 and a high band RF circuit 1904are coupled through a single-pole, double-throw switch 1906 to awide-band TLPF 1908, which may be, for example, of the type shown inFIG. 18. The TLPF 1908 in turn is coupled to an antenna 1910. Byselecting the state of the switch 1906, one of the low band RF circuit1902 and the high band RF circuit 1904 are coupled to the antenna 1910through the TLPF 1908. The TLPF 1908 is electrically tuned to a cut-offfrequency suitable to the selected RF circuit, thereby allowing a singlewide-band TLPF to filter unwanted frequencies relative to the frequencyband of the selected RF circuit (the control circuitry is not shown).While FIG. 19 shows only two RF bands, the concepts embodied in theillustration extend to more than two RF bands by using a multi-throwswitch 1906 and a TLPF 1908 with a suitably wide tuning range.

The teachings above with respect to tunable low pass filters apply aswell to tunable high pass filters (THPFs), as would be apparent to oneskilled in the art. Referring to FIG. 18, the TLPF shown can beconverted to a THPF by removing inductor 1802 b. Of course, other highpass filter designs may be useful for various applications, so long asthey are tunable. Both tunable low pass filters and tunable high passfilters are useful, for example, in RF duplexers and diplexers. Adiplexer is typically used to separate high band and low band signalsfrom a single antenna. This typically involves large frequencyseparation between the high frequency band and the low frequency band.Note that both transmit and receive signals flow through the highfrequency section and the low frequency section, so the diplexer doesnot have an impact on the duplexing method. A duplexer is typically usedto isolate proper transmission and receive paths from one common antennapath, and allows frequency division duplexing, where transmission andreception occur simultaneously on a common antenna. In both cases, thebasic circuitry is the same, although duplexer's typically require“sharper” filters, making them more difficult to design in integratedcircuit form. The discussion below focuses on duplexers, but applies aswell to diplexers.

Duplexers typically include a high pass filter and a low pass filter andneed to cover a wide range of frequency bands, which increases thechallenge of designing such duplexers using fixed filtering elements,especially in a miniature integrated form. In a typical RF front-endarchitecture, a fixed duplexer cannot impedance match well with theantenna in all of the supported frequency bands, and thereforeadditional impedance matching network elements are needed. Such problemsare resolved by a tunable duplexer that includes both a TLPF forfiltering a low-band RF path and a THPF for filtering a high-band RFpath. The cut-off frequencies (and most signal rejection notches) ofboth tunable filters can be shifted as needed by means of a controlinterface to accommodate signals in different frequency bands. As anexample, FIG. 20 is a block diagram of one embodiment of an RF duplexer2002 incorporating both a tunable low pass filter and a tunable highpass filter. An antenna 2004 is coupled through a TLPF 2006 to a lowband RF circuit 2008, and through a THPF 2010 to a high band RF circuit2012. The combination of the TLPF 2006 and the THPF 2010 effectivelyform a tunable notch filter to provide band rejection for the RFfrequencies between the low band and the high band.

As would be apparent one of skill in the art, the teachings above withrespect to tunable notch filters also apply to tunable bandpass filters.A tunable bandpass filter may implemented as a low pass filter, such asthe type shown in FIG. 12, coupled in series to a capacitor, with one ormore of the inductors or capacitors implemented with tunable components,as described above. Alternatively, a THPF and a TLPF can be combined soas to effectively form a tunable bandpass filter to provide bandadmittance. One or more tunable bandpass filters may be used in variousRF circuits where it is more advantageous to admit rather than reject aselectable band of frequencies.

As noted previously, an important aspect of the present invention isthat the tunable notch or bandpass filters or tunable low or high passfilters are fabricated within the same IC package as the associatedfrequency based circuitry; in the examples shown in the accompanyingfigures, such circuitry comprises an RF switch, but other circuitry maybe used. Such integration reduces or eliminates package and printedcircuit board (PCB) RLC parasitic values, and also allows residual andother parasitic capacitance, inductance, and resistance in theassociated circuitry and package to be absorbed and compensated. Forexample, such integration, particularly if a DTC is also integrated onthe same chip to provide the desired tunability, reduces parasiticcapacitances from such sources as IC pad shielding “cages”,electrostatic discharge (ESD) circuits, and the IC package itself.Importantly, integration of tunable filters in an IC results in a totalinsertion loss that is less than the total insertion loss that wouldexist if the tunable filters were external to the IC. Accordingly, theinvention encompasses co-designing a frequency based circuit and one ormore tunable filters such that the overall performance of the integratedcombination is better than the simple combination of separate componentsperforming the same functions.

Other advantages of integration of tunable filters is that they canhandle large signal levels and have low distortion, which arerequirements for use in many RF circuits, such as an RF radio front end.

As should be readily apparent to one of ordinary skill in the art,various embodiments of the invention can be implemented to meet a widevariety of possible frequency suppression specifications. Thus,selection of suitable R, L, and C values (taking into account parasiticvalues resulting from IC implementation) are a matter of design choice.The switching and passive elements may be implemented in any suitable ICtechnology, including but not limited to MOSFET and IGFET structures.Integrated circuit embodiments may be fabricated using any suitablesubstrates and processes, including but not limited to standard bulksilicon, silicon-on-insulator (SOI), and silicon-on-sapphire (SOS)processes.

Another aspect of the invention includes a method for selectivelyfiltering unwanted frequencies from a signal path of a frequency basedintegrated circuit, including the steps of:

STEP 1: fabricating at least one tunable filter and a frequency basedcircuit in an integrated circuit package;

STEP 2: coupling at least one tunable filter to a signal path of thefrequency based circuit;

STEP 3: configuring the frequency based circuit and each tunable filterto absorb and compensate for residual and other parasitic impedancepresent in the frequency based circuit integrated;

STEP 4: selectively activating at least one coupled tunable filter tofilter unwanted frequencies from the signal path of the frequency basedintegrated circuit.

Another aspect of the invention includes methods for coupling,configuring, and operating circuit elements as shown in the variousfigures and described above.

A number of embodiments of the invention have been described. It is tobe understood that various modifications may be made without departingfrom the spirit and scope of the invention. For example, some of thesteps described above may be order independent, and thus can beperformed in an order different from that described. It is to beunderstood that the foregoing description is intended to illustrate andnot to limit the scope of the invention, which is defined by the scopeof the following claims, and that other embodiments are within the scopeof the claims.

What is claimed is:
 1. A tunable capacitor compensated filter,including: (a) a tunable filter configured to be selectively coupled toa signal path of a frequency based circuit to filter unwantedfrequencies from the signal path, the tunable filter having an effectivecapacitance when the tunable filter is coupled to the signal path; and(b) a bypassable compensation capacitor configured to be selectivelycoupled to the signal path when the tunable filter is not coupled to thesignal path, the bypassable compensation capacitor having a capacitancevalue to approximately match the capacitive value of, and substitutefor, the effective capacitance of the tunable filter.
 2. The tunablecapacitor compensated filter of claim 1, wherein the tunable filterincludes one or more of a tunable notch filter, a tunable bandpassfilter, or a tunable low pass filter.
 3. The tunable capacitorcompensated filter of claim 1, wherein the tunable filter includes atleast one tunable RLC filter.
 4. The tunable capacitor compensatedfilter of claim 1, wherein the tunable filter includes at least onetunable RLC filter having at least one tunable capacitor C.
 5. Thetunable capacitor compensated filter of claim 4, wherein at least onetunable capacitor C of the at least one tunable capacitor C is digitallytunable.
 6. The tunable capacitor compensated filter of claim 1, whereinthe tunable filter includes at least one tunable RLC filter having atleast one tunable inductor L.
 7. The tunable capacitor compensatedfilter of claim 6, wherein at least one tunable inductor L of the atleast one tunable inductor L is digitally tunable.
 8. The tunablecapacitor compensated filter of claim 1, further including a tuninginductor coupled in series in the signal path.
 9. The tunable capacitorcompensated filter of claim 1, wherein the signal path includes a commonport and at least one selectable port coupled to the signal path, eachselectable port being switchable between an active state and anon-active state.
 10. A tunable capacitor compensated filter, including:(a) a tunable filter configured to be selectively coupled to ground and,through a first switch, to a signal path of a frequency based circuit tofilter unwanted frequencies from the signal path, the tunable filterhaving an effective capacitance when the tunable filter is coupled tothe signal path through the first switch; and (b) a bypassablecompensation capacitor configured to be selectively coupled to groundand, through a second switch, to the signal path through the secondswitch when the tunable filter is not coupled to the signal path throughthe first switch, the bypassable compensation capacitor having acapacitance value to approximately match the capacitive value of, andsubstitute for, the effective capacitance of the tunable filter.
 11. Thetunable capacitor compensated filter of claim 10, wherein the tunablefilter includes one or more of a tunable notch filter, a tunablebandpass filter, or a tunable low pass filter.
 12. The tunable capacitorcompensated filter of claim 10, wherein the tunable filter includes atleast one tunable RLC filter.
 13. The tunable capacitor compensatedfilter of claim 10, wherein the tunable filter includes at least onetunable RLC filter having at least one tunable capacitor C.
 14. Thetunable capacitor compensated filter of claim 13, wherein at least onetunable capacitor C of the at least one tunable capacitor C is digitallytunable.
 15. The tunable capacitor compensated filter of claim 10,wherein the tunable filter includes at least one tunable RLC filterhaving at least one tunable inductor L.
 16. The tunable capacitorcompensated filter of claim 15, wherein at least one tunable inductor Lof the at least one tunable inductor L is digitally tunable.
 17. Thetunable capacitor compensated filter of claim 10, further including atuning inductor coupled in series in the signal path.
 18. The tunablecapacitor compensated filter of claim 10, wherein the signal pathincludes a common port and at least one selectable port coupled to thesignal path, each selectable port being switchable between an activestate and a non-active state.